This invention relates to an improved system for controlling the tilting of a mast assembly and the lifting of the fork of materials handling vehicles.
In several types of prior art materials handling vehicles, a mast assembly is provided which is capable of tilting in a forward direction to facilitate the loading or unloading of the materials carried by the vehicle. A pair of lift forks are carried by the mast assembly for lifting and carrying the materials.
In these vehicles, it is common to provide a single motor driven pump for providing hydraulic pressure to control the tilt of the mast assembly and also to control the lifting or lowering of the forks and perhaps accessory functions, such as moving the forks from side to side or for grasping loads, such as barrels.
The maximum forward tilt of the mast assembly is limited by the mechanical construction of the vehicle, but for safety reasons, there is usually provided a forward tilt limit beyond further tilting will not be permitted if the forks were also above their specified limit. This is shown in FIG. 2 as zone 15. The tilt limit is established by a limit switch which is actuated when the forward tilt reaches or exceeds a preset angle. Actuated merely means that the condition of the switch has changed after the event, either from open to closed or from closed to open.
The maximum height of the forks is also naturally limited by the physical structure of the mast assembly, but again for safety reasons, the forks will be limited in the height they can lift a load above the ground if the forward tilt of the mast assembly is beyond its predetermined limit. The height limit is established by a limit switch which is actuated by the fork assembly at a predetermined intermediate height.
As shown in FIG. 2, if the forks are between the ground and the height limit, as represented by line HL, then the forward tilt of the mast assembly will not be restricted; it may be tilted forward until it reaches its maximum tilt angle. Similarly, if the forward tilt of the mast assembly is between 0.degree. and the tilt limit as represented by line TL, then the height to which the forks can be raised will not be limited by the system but can be raised to their maximum height. When either the forward tilt limit or the height limit has been exceeded, the vehicle will be operating in either the Tilt Limited Zone 20 or the Height Limited Zone 25.
No operation of the tilt or lift functions is permitted that would place the vehicle in the area or zone 15. A no restriction zone is established when neither limit has been exceeded, as illustrated by the area 10.
In prior art vehicles, if both of the limit switches have been activated, it would not be possible to continue either the forward tilt of the mast or the raising of the forks since operation of the single motor driven pump is usually terminated. It is always possible, however, to move the mast rearward or lower the forks, thus deactivating one or both of the switches.
As shown in FIG. 2, if the vehicle is in the height limited zone 25 and the forks are raised, when the height limit is reached, as represented by the point 26 on line HL, further raising is prohibited, but it should be permissible to continue to forward tilt the mast assembly, but in prior art vehicles, this is possible only after the forks have been lowered slightly, enough to release the fork limit switch, in order to forward tilt. Similarly, if the vehicle is in the tilt limited zone 20 and the mast is moved forward to its limit at point 22, then continued raising of the forks should be permissible, but prior art vehicles require that the forward tilt angle be reduced enough to release the tilt limit switch before such continued operation is permitted.
This type of operation of the prior art vehicles is a waste of time and requires additional manipulation of the controls, but it is also clear it was a result of concern that the vehicle not be permitted to operate in the restricted region 15.